In a watch circuit, power consumption is critical because battery life and battery size are related to the power consumption of the circuit. Generally, the less power required, the smaller the allowable battery size and the longer a battery of a given size will last.
Watch circuits typically include an oscillator for producing periodic pulses at a stable frequency, and a chain of frequency divider stages for dividing the oscillator frequency down to a convenient periodic time reference, such as one pulse per second. The one pulse per second signal drives timekeeping circuitry which in turn provides signals to the watch display.
Since the oscillator and the first few divider stages of the frequency divider chain operate at the highest frequency of the circuit, a major percentage of the total circuit power consumption occurs there. A significant amount of the power consumption is due to the operation of the oscillator alone.
The power consumption of the oscillator can be reduced by reducing the gain of the amplifier used in the regenerative feedback loop of the oscillator. However, if the oscillator amplifier gain is reduced so that a significant reduction in operating current is realized, then the time required for the oscillator to start when battery power is initially applied to the circuit becomes excessively long, or the oscillator may fail to start at all. Since the starting time of the oscillator is partially dependent on external capacitors used for trimming the oscillator frequency, the starting time, or the failure to start, varies with each individual watch circuit. Preferably, the starting time should be no longer than 3 seconds in order to expedite testing of watch assemblies during manufacture, and to be acceptable to the consumer.
It is known to provide an oscillator initiation circuit which controls the gain of an oscillator so that the oscillator has a high gain at the time of initiation in order to provide a shortened period of oscillator initiation time, and thereafter has a lower gain in order to avoid excessive power consumption. For example, see U.S. Pat. No. 4,039,973 to O. Yamashiro which discloses an initiation circuit in a crystal controlled oscillator.